[Codegen] Allow default value for non nullable variables

[martinbonnin]

See #2686. This allow to omit the variable value if it has a defaultValue (even if the variable is non-nullable)

query launches($id: String! = "83") {
  launch(id: $id) {site}
}

will generate

// by default, id will be omitted 
data class LaunchesQuery(val id: Input<String> = Input.absent()) { ... }

I'm opening this mainly for reference. That's a potentially relatively seriously invasive/breaking change that has bitten us in the past (#2744, #2742). Given that it can be worked around relatively easily by passing the defaultValue in Kotlin code, and unless there are other important use cases I'm missing, this PR will only be merged for the next major version

[martinbonnin]

Closing for now. Will reopen for 3.0